The fuel mixture in the engine needs to complete combustion at around 10° after top dead center. At idle the engine needs little advance from about TDC to achieve the desired end of combustion at 10° ATDC. As the engine speed increases the time required for the fuel to burn remains somewhat constant. The crankshaft is turning faster so the piston arrives at 10° ATDC sooner. The mechanical advance moves the timing so the the end of combustion will still remain at 10° ATDC. Under light loads the throttle plate in the carburetor is only slightly open resulting in high manifold vacuum. The means that the cylinders will not receive as much fuel mixture resulting in lower compression. The lowered compression results in the fuel molecules being farther apart resulting in increasing the burn time of the fuel mixture. A vacuum advance is controlled by manifold vacuum and will increase the timing in addition to the mechanical advance again keeping the end of combustion at 10° ATDC. When the engine load increases the throttle plates open more causing the manifold vacuum to drop and allowing more air/fuel mix to enter each cylinder. This increases the compression in the cylinder pushes the individual fuel molecules closer together which in turns causes the fuel mixture to burn faster. As the manifold vacuum drops off the vacuum advance also retards the timing to again keep the end of combustion in the proper place.
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Today's Featured Article - Grain Threshing in the Early 40's - by Jerry D. Coleman. How many of you can sit there and say that you have plowed with a mule? Well I would say not many, but maybe a few. This story is about the day my Grandfather Brown (true name) decided along with my parents to purchase a new Ford tractor. It wasn't really new except to us. The year was about 1967 and my father found a good used Ford 601 tractor to use on the farm instead of "Bob", our old mule. Now my grandfather had had this mule since the mid 40's and he was getting some age on him. S
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